The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 (typically MnPt between about 150 and 200 Å thick) whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A capping layer 18 usually lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Current read head structures are, as shown in FIG. 1, based on the exchange bias pinned synthetic spin valves. It is understood that in order to improve GMR, current shunting reduction and better structure coherence are needed. There are, however, major drawbacks to this structure. Due to the significant reduction of MnPt thickness, there is no Hex, thus no clear magnetic configuration, which makes it impossible to evaluate the GMR behavior on the full film level and/or during wafer process. In addition, the stress-induced anisotropy is around 5,000 e only. Considering stress variations from device to device, this may not be sufficient to avoid pin rotation or pin reversal related head degradation in the drive environment.
To solve this problem, we introduce a modified GMR structure with enhanced He and/or Hex along with a new annealing sequence to promote a clean magnetic configuration in the AP pinned layers, which enables us to measure the GMR behavior at the full film level and during wafer processing. This new annealing sequence. can be readily incorporated into the current standard wafer process sequence. It is suitable, not only for the self-pinned SV structure, but is also applicable to exchange bias based SV.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. Nos. 6,655,008 and 6,219,208 and in U.S. Patent Application Publication 2001/0004798, Gill shows IrMn as a possible AFM layer (around 200 Å thick) in a self-pinned spin valve. Also shown is a specular reflector layer of Cu, Ag, or Au to overcome loss of conduction electrons. A synthetic AFM is not used. In U.S. Patent Application Publication 2003/0179515, Pinarbasi discloses a self-pinned spin valve where PtMn is preferred as the AFM because IrMn is corrosive.
In U.S. Patent Application Publication 2003/0179516 Freitag et al. use MnPt as the AFM layer while in U.S. Patent Application Publication 2003/0218903, Luo describes a self-pinned spin valve where the AFM is very thin or not deposited at all. IrMn is mentioned as a possible replacement for PtMn.